Faulted circuit detector having isolated indicator

ABSTRACT

A faulted circuit detector having an electrically isolated and remotely positioned indicator detects fault currents in a monitored conductor. When a fault of a predetermined magnitude is detected, a light pulse is transmitted from the detector to the indicator via a fiber optic cable where, upon receipt, the pulse is converted to an electrical pulse causing the indicator to indicate a &#34;fault&#34; state. A reset circuit within the indicator resets the indicator at regular intervals to the &#34;normal&#34; state as long as a predetermined minimum voltage is present on the monitored conductor. The detector includes a temperature compensation circuit and low pass filter circuit so as to prevent misoperation due to varying ambient temperatures or high frequency transients. A test circuit is also provided to allow service personnel to field-test the device with the monitored conductor in an energized state.

This application is a continuation-in-part of Ser. No. 543,223 filedJun. 25, 1990, now issued as U.S. Pat. No. 5,159,319.

BACKGROUND OF INVENTION

The present invention relates generally to fault sensors and indicatorsfor electrical distribution systems. More particularly, the inventionrelates to self-resetting fault indicators, wherein after the occurrenceof a fault on a monitored line, the indicator is reset to display a"normal" indication in instances where the fault or system disturbancewas of a transient nature. Still more particularly, the inventionrelates to a fault detector having a self-resetting fault indicator thatis electrically isolated from the sensor assembly and in communicationwith the sensor via a fiber optic link.

Fault detectors of various types have been employed for detecting faultsin electrical power distribution systems and for providing a visualindication that such a fault has been detected. Such detectors typicallyinclude a sensor and an indicator. The sensor is usually connected to aload carrying conductor for detecting the presence of a fault or systemdisturbance in the monitored conductor and for signalling the indicatorof such an event. The sensor typically includes a clamp-on device whichclamps directly over the conductor that is to be monitored. Other priorart sensors have been mounted on test points provided on connectors orcomponents of the distribution system. The indicator is electricallyconnected to the sensor and is often mounted remotely from the sensor soas to provide a more convenient observation point for workmen. Uponreceipt of a signal that a fault of a predetermined magnitude hasoccurred, the indicator displays a visual indication that a fault ordisturbance has been detected in the monitored line.

Fault detectors are typically installed on each phase of the variousbranches of an electrical distribution circuit so as to provideinformation for repair crews who must find and repair faulted circuitswhen they occur. Without fault indicators, the repair crews must operateon a trial and error basis in order to find the faulted branch circuit.This may be done by disconnecting the individual branch circuits, one ata time, from their common feeder circuit, and then closing the feedercircuit breaker that supplies the network of branch circuits so as todetermine if the isolated or disconnected branch was the one in whichthe fault occurred. If the fault still exists on the system, electricalrelays or other protective devices will automatically cause the feedercircuit breaker to "trip," thereby again opening the feeder circuit.This will indicate to the repair crew that the fault was not on thedisconnected branch, but instead is on one of the branch circuits stillconnected to the feeder circuit. This trial and error approach tofinding the faulted circuit is eliminated through the use of faultedcircuit indicators, as the repair crews need only visually inspect theindicators and locate the line or lines having indicators displaying a"fault" indication.

On lines having faulted circuit detectors, after the malfunction orfault has been located and repaired, the indicators must be reset fromtheir "fault" to their "normal" indication state. Many prior artindicators had to be manually reset using a nonconductive tool known inthe art as a "hot stick". other fault detectors have included means forautomatically resetting the indicator to a "normal" state once thenormal or steady-state load current has existed for a predeterminedlength of time.

Self-resetting fault detectors typically employ a mechanical flag orother visual display device, a trip circuit for causing the displaydevice to indicate a fault upon the occurrence of a current of apredetermined magnitude in the monitored conductor, and aperiodically-actuated reset circuit for causing the display device tomove to its reset or "normal" state upon the reoccurrence of normalsteady-state load current in the monitored conductor.

Because the sensors are often mounted in relatively inaccessiblelocations, it is often desirable that the indicator be located remotelyfrom the sensor so as to provide repair crews a better vantage pointfrom which to visually check the indicator. In these instances, thesensor and indicator portions of the faulted circuit detector havetypically been connected by an electrical conductor or conductors. In atypical application, such sensors are mounted on the primary or highvoltage side of a distribution transformer, while the indicator ispositioned remotely, on the low voltage or secondary side. Having thesensor and remote indicator connected by an electrical conductorpresents the undesirable situation that the conductor's insulation couldbreak down and cause a fault to ground or to another phase. The previousmethods used to isolate the high voltage side sensor from the lowvoltage side indicator have included the use of a conductor formed of acarbon impregnated material. Such a conductor has an extremely highimpedance, and thus imposes significant limitations on thefunctionability of the indicator. Further, the wire is nevertheless aconductor of electric current, and may still provide a current path toground or to the secondary voltages.

Present-day fault detectors have additional disadvantages and sometimesprove unreliable, such as by falsely indicating the presence of a faultor failing to indicate that a fault has occurred. Such misoperation maybe caused, for example, by the presence of high frequency transients onthe circuit that is being monitored by the detector. Such transients aretypically caused by switching events occurring elsewhere on theelectrical system. Misoperation of the detector may also occur becausethe sensors employed in many present-day detectors are affected to agreat degree by changes in temperature and because the detectors aretypically installed in outdoor locations where they are subjected to awide range of ambient temperatures.

Thus, although there presently exists apparatus for detecting andindicating the presence of faults on power system circuits, it should beapparent that such apparatus may be improved both to enhance the degreeof isolation between the sensor assembly and the indicator, and toenhance the operational reliability of the faulted circuit detector.

SUMMARY OF THE INVENTION

Accordingly, there is provided herein an apparatus for sensing anovercurrent or fault condition in a monitored electrical conductor andproviding a visual indication of such condition at a remotely located,electrically isolated indicator. The apparatus of the present inventiongenerally comprises a sensor for detecting a fault condition in themonitored conductor and an indicator for providing a visual indicationthat a fault has occurred. The invention further includes acommunication path between the sensor and indicator, such pathcomprising an electrical insulator, such as a fiber optic cable, suchthat the sensor and indicator are electrically isolated from one anotherso as to prevent fault currents from being transmitted therebetween.Also provided is an automatic means for resetting the indicator to itsnormal or nonfault indication state as long as a predetermined minimumvoltage is present on the monitored electrical conductor.

The sensor may include a transmitter circuit comprising a light emittingdiode (LED) adapted for transmitting a light pulse through the fiberoptic cable when current is permitted to flow through the LED, and asilicon unilateral switch which closes, thereby allowing current to flowto the LED, when a predetermined voltage is present across the switch.The sensor's transmitter circuit may further comprise a programmableresistor for causing the predetermined voltage to appear across thesilicon unilateral switch when a fault current of a predeterminedmagnitude has been sensed in the monitored conductor. To prevent highfrequency transients from causing the transmitter circuit to falselysignal the indicator that a fault has been detected, the sensor mayinclude a low pass filter for excluding those transients that have ahigh enough magnitude to otherwise cause the transmitter circuit tooperate. Additionally, the sensor may include a temperature compensationcircuit designed to minimize the effect that temperature change mightotherwise have on the sensor output, thereby increasing the accuracy andreliability of the invention.

The automatic reset means in the present invention may include adetector circuit for determining when the predetermined minimum voltageis present on the monitored conductor, a reset circuit for continuouslyresetting the indicator at periodic intervals as long as the minimumvoltage is detected, and a holdoff circuit for deactivating the resetcircuit when the predetermined minimum voltage is not detected on themonitored conductor. The indicator may further include a fiber opticdetector for receiving the light pulse transmitted through the fiberoptic cable and converting the pulse to an electrical pulse. Theindicator may also comprise a timer, such as a dual CMOS timer package,which receives the electrical pulse from the fiber optic detector andtransmits an output pulse to cause the indicator to change from a"normal" to a "fault" indication state, and to cause the holdoff circuitto delay the reset circuit a predetermined period of time beforeresetting the indicator.

The apparatus may further include a test circuit for testing theapparatus and causing the indicator to indicate the presence of theovercurrent condition when such a condition does not actually exist onthe monitored conductor. Such a test circuit may comprise a magneticallyoperable switch positioned in the sensor which can be actuated bymanually positioning a magnet in close proximity to the sensor, therebyallowing maintenance or service personnel to safely check the operationof the apparatus while the monitored conductor remains in its"energized" or load-carrying state.

Thus, the present invention comprises a combination of features andadvantages which enable it to substantially advance the fault detectortechnology by providing a self-resetting indicator that is electricallyisolated from the fault sensing apparatus and which can be field testedwithout removing the monitored conductor from service. The inventionalso includes circuitry in the sensor assembly which serves to insurereliability and accuracy by preventing high frequency transients andvarying ambient temperatures from causing misoperation of the faultdetector. These and various other characteristics and advantages of thepresent invention will be readily apparent to those skilled in the artupon reading the following detailed description and referring to theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

For an introduction to the detailed description of the preferredembodiment of the invention, reference will now be made to theaccompanying drawings, wherein:

FIG. 1 shows a perspective view of the faulted circuit detector of thepresent invention;

FIG. 2 shows a schematic diagram of the sensor assembly of the detectorshown in FIG. 1;

FIG. 3 shows a schematic diagram of the indicator assembly of thedetector shown in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring initially to FIG. 1, there is shown one example of a faultedcircuit detector 10 structured in accordance with the principals of thepresent invention. Detector 10 generally comprises a sensor assembly 12and an indicator assembly 14. In the example of the preferred embodimentshown, sensor assembly 12 is disposed about a load-carrying monitoredconductor 18 which is to be monitored for fault currents. Indicatorassembly 14 is positioned remotely from sensor assembly 12 and is incommunication therewith by means of fiber optic cable 16.

Sensor Assembly

Referring now to FIG. 2, there is shown a schematic diagram of thecircuit for the sensor assembly 12 shown in FIG. 1. In general, sensorassembly 12 includes a current transformer 20 for sensing the currentlevels in conductor 18 (FIG. 1), and a transmitter circuit 22 fortransmitting a light pulse to indicator assembly 14 when a predeterminedcurrent level is sensed in the monitored conductor 18. For improvedreliability and accuracy, sensor assembly 12 further includes atemperature compensation circuit 150 for negating the adverse effectsthat temperature changes can have on the operation of currenttransformer 20, and a low pass filter 160 for filtering out undesirablehigh frequency transients which would otherwise be passed to transmittercircuit 22, possibly causing a false indication of a fault condition.

As shown in FIG. 2, current transformer 20 comprises a wire coil 21having output leads 24, 26. When leads 24 and 26 are connected to aload, a current proportional to the current flowing in monitoredconductor 18 is induced in coil 21 and communicated to the load vialeads 24, 26. In the preferred embodiment, coil 21 is comprised ofcopper wire. A suitable current transformer 20 for the present inventionincludes a coil 21 consisting of 7,000 turns of No. 41 gauge copper wirewound on a plastic bobbin core. The nominal direct current resistance(DCR) measured across coil 21 is 1,370 ohms ±15%.

The DCR of copper wire will change as its temperature changes. Morespecifically, as the temperature of the wire increases, the DCR willalso increase in a linear fashion. A material which exhibits thisproperty is said to have a positive temperature coefficient (PTC). Thus,without any temperature compensation means, as the temperature of coil21 increases in response to an increase in ambient temperature, theresistance of coil 21 also increases, thus causing a disproportionallysmaller current than the current flowing in monitored conductor 18 topass to transmitter circuit 22. This situation could cause misoperationof the detector.

Accordingly, the sensor assembly 12 includes temperature compensationcircuit 150 which generally includes a thermistor 152 and a resistor154. As shown in FIG. 2, thermistor 152 is wired electrically inparallel with resistor 154. One end of this parallel combination iselectrically connected to lead 24 of current transformer 20 atconnection 27 while the other end is tied to low pass filter 160 atconnection 161.

Thermistors may have either positive or negative temperaturecoefficients. In this application, thermistor 152 has a negativetemperature coefficient (NTC) which means that its resistance willdecrease as its temperature increases. A thermistor found suitable forthis application is manufactured by Fenwal Electronics, Inc., Model No.197-102DAG-A01, and has a resistance of 1,000 ohms at 25° C. ±10% andhas a resistance ratio of 6.35. The resistance ratio of a thermistor isthe ratio of its change in resistance to the change in temperatureexperienced by the thermistor. To ensure that thermistor 152 is exposedto the same temperature as coil 21, thermistor 152 is physically placeddirectly on top of, and tape-wrapped to, coil 21.

In the preferred embodiment as shown in FIG. 2, resistor 154 is rated825 ohms, 1/4 W, 1% tolerance. Resistor 154 is selected to have anegligible resistance change over the predetermined operating range oftemperatures, such temperature range being approximately -40° C. to +85°C. By connecting resistor 154, which is substantially unaffected bytemperature changes, in parallel with thermistor 152, which has anegative temperature coefficient, the resistance of the parallelcombination of thermistor 152 and resistor 154 will decrease withincreased temperatures. In this manner, thermistor 152, resistor 154 andcoil 21 are selected based upon their resistances and temperaturecoefficients so that, with changes in temperature, the resulting changein resistance of coil 21 will be negated by an opposite and proportionalchange in the resistance of the parallel combination of thermistor 152and resistor 154. Thus, irrespective of any temperature change withinthe prescribed range, the voltage appearing across capacitor 162 of thelow pass filter 160 will remain nearly constant for a given current inmonitored conductor 18. This result provides a consistent output to thetransmitter circuit 22 over a wide temperature range, thereby insuringgreater accuracy and reliability in fault indicator performance.

Referring still to FIG. 2, low pass filter 160 generally comprisescapacitors 162, 164 and inductor 166. As described above, the outputfrom temperature compensation circuit 150 is connected across capacitor162. Low pass filter 160 is employed to filter or remove high frequencytransients from the signal communicated to it by temperaturecompensation circuit 150. Such transients, which may be caused by avariety of switching events or lightning strikes, for example, typicallycause high magnitude currents to flow in monitored conductor 18, whichin turn induces a corresponding high magnitude current in coil 21 ofcurrent transformer 20. This current, if allowed to pass to transmittercircuit 22, could cause the detector 10 to falsely indicate the presenceof a fault. The low pass filter 160 is included to distinguish betweentransient events such as these and a fault-induced overcurrent so as toensure proper operation of the faulted circuit detector 10. Capacitors162, 164 and inductor 166 of the low pass filter circuit 160 areselected to allow only a narrow range of frequencies to flow through thelow pass filter 160 to transmitter circuit 22. The band of frequenciesthat low pass filter 160 is designed to pass is centered about thenominal system frequency of the conductor being monitored, i.e., 60 hzin the United States. In the preferred embodiment, the filter's roll offpoint may be within the range of approximately 1,000-2,000 hz.

After the signal has been conditioned via the low pass filter circuit160, it passes through a bridge rectifier circuit 28 of transmittercircuit 22. The bridge rectifier circuit 28, which is connected directlyto the output of low pass filter circuit 160, rectifies the alternatingcurrent to dc for use by the transmitter circuit 22.

Connected across coil 21 of the current transformer 20 is varistor 30which provides over-voltage protection for the circuitry in sensorassembly 12. If the current induced in coil 21 of current transformer 20approaches a predetermined level, the voltage appearing across coil 21,varistor 30 and capacitor 162 will reach the breakdown voltage of thevaristor (22 volts in the preferred embodiment), at which point thevaristor 30 will act as a conductor, shunting coil 21 and preventingdamage to the circuitry of sensor assembly 12.

At normal voltages, below the breakdown voltage of varistor 30, currentleaving the bridge rectifier 28 is cascaded through the seriescombination of programming resistor 32 and transistor 34 which providethe load for current transformer 20. Nominally, the voltage across thecombination of resistor 32 and transistor 34 is between 0 and 12 volts.Connected in parallel with the series combination of programmingresistor 32 and transistor 34 is capacitor 36 which is used forfiltering and for energy storage as explained in more detail below.

Also in parallel with the series combination of resistor 32 andtransistor 34 is a series combination comprising silicon unilateralswitch (SUS) 38, fiber optic transmitter or LED 40 and resistor 42. SUS38 has the characteristic that as the voltage across it rises, itmaintains a high resistance until a predetermined level is reached. Atthat predetermined voltage, which is equal to 8.2 volts in the preferredembodiment, SUS 38 becomes conductive and turns "on", allowing currentto flow therethrough to fiber optic transmitter 40. Resistors 44 and 46form a voltage divider network that is employed in the circuit 22 tobias the SUS 38 to the proper operating condition.

Resistor 32 is a resistor which is used to adjust the amount of currentwhich will trigger the SUS 38. The SUS 38 will become conductive at 8.2volts. Thus, the larger resistor 32 is, the less current that isnecessary to trigger SUS 38. When SUS 38 becomes conductive, capacitor36 discharges through SUS 38, fiber optic transmitter 40 and resistor42. The capacitor 36 preferably stores a relatively small amount ofenergy, which, when discharged, provides a surge to the transmitter 40.Transmitter 40 thereafter responds by providing a high intensity butshort pulse of light through fiber optic cable 16. When the currentthrough SUS 38 drops back toward zero upon discharge of capacitor 36,SUS 38 switches back to its "off" or highly resistive mode, and thevoltage is allowed to build up again on capacitor 36 and programmingresistor 32. Resistor 42 is provided to limit the discharge current ofcapacitor 36 through fiber optic transmitter 40 so that transmitter 40is not damaged by excessive current levels.

In general, the transmitter circuit 22 converts a small amount ofcurrent generated by current transformer 20 into a voltage level. Whenthe voltage level exceeds a predetermined value, 8.2 volts in thisembodiment, capacitor 36 discharges through SUS 38 and fiber optictransmitter 40 which, in turn, provide a bright, single pulse of light.

The preferred embodiment of transmitter circuit 22 further includes atest circuit comprised of resistor 48, magnetic reed switch 50 andtransistor 34. Resistor 48 has a resistance that is larger byapproximately an order of magnitude than the resistance of programmingresistor 32. Reed switch 50 is positioned within sensor assembly 12 suchthat when a repairman or operator positions a magnet near the sensorassembly 12, such as by use of a "hot stick" magnetic reed switch 50closes, thereby turning off transistor 34 and effectively removingresistor 32 from the circuit. Because the resistance of resistor 48 ismuch larger than the resistance of programming resistor 32, the voltageacross resistor 48 rises rapidly. When the threshold voltage of SUS 38is reached, SUS 38 becomes conductive, triggering fiber optictransmitter 40 to fire a high intensity pulse of light. It will beunderstood by those skilled in the art that the test circuit comprisedof transistor 34, reed switch 50 and resistor 48 could all be deletedfrom the circuit if it was not desired that the fault detector 10 havethe capability of being manually tested. In such a configuration,programming resistor 32 would be tied directly to ground bus 52 ratherthan being connected to transistor 34. The operation of the transmittercircuit 22 would then be identical to that previously described.

Fiber Optic Transmission Cable

In the preferred embodiment, a fiber optic transmission cable 16 is usedas the transmission link between the sensor assembly 12 and indicatorassembly 14. Any conventional fiber optic cable can be used, as long asit provides electrical isolation from the adjacent electricaldistribution system. Preferably, the cable has a length of approximatelysix feet to permit the indicator assembly 14 to be strategically locatedfor easy viewing.

Indicator Assembly

Referring now to FIG. 3, there is shown a schematic diagram of thecircuit for the indicator assembly 14, the circuit generally comprisingan indication circuit 54, a reset circuit 56, a hold-off circuit 58 anda power supply circuit 60.

Power Supply Circuit

The control or input power for the indicator assembly 14 is suppliedfrom the monitored conductor 18, such as by means of a control powertransformer 140 having its primary side connected to conductor 18, suchthat the transformer's secondary side supplies the input voltage to theindicator assembly 14. In this configuration, the input voltage to theindicator is directly proportional to the voltage on the monitoredconductor 18. As shown in FIG. 3, alternating current is supplied fromthe control power transformer 140 to indicator assembly 14 through leads130, 132. A varistor 114 is provided and is connected across the inputleads to provide over-voltage protection for the indicator assembly 14.Should the voltage across leads 130, 132 exceed a predetermined limit,150 volts ac in the preferred embodiment, the varistor 114 will becomeconductive and will shunt the circuits of indicator assembly 14.

The combination of capacitors 116, 118 connected to leads 130, 132comprise a capacitive current limiter. Resistors 120, 122 are placed inparallel with capacitors 116, 118, respectively, and are used to balancethe voltage across the capacitors.

The parallel combination of resistors 120, 122 and capacitors 116, 118are connected to a rectifying bridge circuit 124 which rectifies theincoming ac signal to dc for use by the indicator assembly circuitry.The current flowing to bridge 124 will be limited by capacitors 116, 118to approximately 10 to 15 milliamps ac. This current is rectified to dc,and the output from bridge 124 is approximately 12 volts dc. A resistor126 is placed in parallel with bridge 124 and has a resistance chosen toset the level at which the hold-off circuit 58, described below, willoperate. In the preferred embodiment, resistor 126 is selected so thatthe voltage across it will be 12 volts dc when 60 volts ac is present onthe secondary of control power transformer 140. Capacitor 128 is used tofilter the output from bridge 124 into a smooth dc signal.

Indication Circuit

Referring still to FIG. 3, the pulse of light generated by the fiberoptic transmitter 40 of transmitter circuit 22 (FIG. 2) is transmittedvia fiber optic cable 16 to indicator assembly 14, where it is receivedby fiber optic receiver 62. Receiver 62 is connected between voltage bus134 and ground bus 136 and converts the light pulse into an electricalpulse of equal duration. The output lead 63 of receiver 62 is connectedto an input lead of a dual CMOS timer package 64. The CMOS timer package64 includes two internal timers, a monostable timer 64a and an astabletimer 64b. Monostable timer 64a is employed in the fault indicationcircuit 54, while the astable timer 64b comprises a component of thereset circuit 56, described in more detail below. In general, timer 64ais employed to "stretch" the short duration light pulse received andconverted by receiver 62 into a longer duration electrical pulse. Theduration of the electrical pulse output from timer 64a is predeterminedby the combination of resistor 66 and capacitor 68. Resistor 67 isemployed in the indication circuit 54 to bias the trigger input of timer64a.

Upon receipt by timer 64a of the short duration input pulse fromreceiver 62, the output from terminal OUT2 turns on transistors 70, 72and 74. Resistor 76 is employed as a biasing resistor to set the properoperating voltage for transistor 70. With transistors 70 and 72 turnedon, current is allowed to flow to ground bus 136 through thesetransistors 70 and 72 and through indicator 80, in the direction shownby arrow 82 in FIG. 3. Current flow through indicator 80 in thisdirection causes indicator 80 to change from the "normal" indicationstate to the "fault" or "alarm" state. Indicator 80 is a bistableindicator, thus it will continue to display the "fault" indication untilbeing reset. A varistor 81 is connected in parallel with indicator 80 toprotect indicator 80 from overvoltages. In the preferred embodiment,varistor 81 will become conductive and shunt the current aroundindicator 80 when the voltage across indicator 80 exceeds 22 volts.

Reset Circuit

As set forth above, dual CMOS timer package 64 includes a secondinternal timer 64b that is utilized in reset circuit 56. Generally,reset circuit 56 is composed of timer 64b, resistors 84, 86, capacitor88 and transistors 90, 92, 94 and 96. The combination of biasingresistors 84 and 86, capacitor 88 and transistor 90 forces the timer 64bto execute a ten second delay period and then issue a reset pulsethrough its output terminal, thereby turning on transistors 92, 94 and96. Resistors 98, 100 are included as biasing resistors. Oncetransistors 92 and 94 are turned on, current will flow throughtransistors 92 and 94 to ground bus 136, flowing through indicator 80 inthe direction shown by arrow 102 in FIG. 3. Current in the directionnoted by arrow 102 will cause the indicator 80 to change indications andagain display its "normal" indication. Since the indicator 80 isbistable, it will remain in the "normal" or "reset" state after thereset pulse from timer 64b is over. Timer 64b issues the reset pulseevery ten seconds so that the reset circuit continuously resets, orattempts to reset, indicator 80 to the "normal" state as long as apredetermined voltage is maintained on the monitored circuit, asdetermined by the hold-off circuit 58, described below.

Reset circuit 56 cycles continuously as long as the reset is not delayedby the hold-off circuit 58. To preclude the possibility of simultaneousor nearly simultaneous actuation of the indicator 80 by indicationcircuit 54 and reset circuit 56, and to insure a full 10 second delayalways occurs before indicator 80 is reset, the indication circuit 54,upon occurrence of a fault, actuates the indicator 80 whilesimultaneously actuating the reset circuit 56 by resetting the timer 64bto time zero. More specifically, as stated above, the output from timer64a initiates the long duration pulse upon receipt of a signal fromfiber optic receiver 62. As shown in FIG. 3, the output pulse fromterminal OUT2 of timer 64a, which turns on transistor 70, 72 and 74,also turns on transistor 90 which will force the reset timer 64b tostart out from time zero. In this manner, the resetting of indicator 80will always be delayed ten seconds after a "set" pulse is generated bytimer 64a.

Hold-Off Circuit

The indicator assembly 14 also includes a hold-off circuit 58 fordelaying the reset function if there is insufficient voltage on themonitored conductor 18. As shown in FIG. 3, the voltage available toindicator assembly 14 from the secondary side of control powertransformer 140 will be proportional to the voltage on monitoredconductor 18. In the preferred embodiment, the reset circuit 56 will bedisabled by the hold-off circuit 58 unless at least 87 volts isavailable to indicator assembly 14. As a result, the fault indicator 80is reset by reset circuit 56 to indicate a "normal" state if more than87 volts appears across leads 130 and 132. If less than 87 volts arepresent, the fault indicator 80 is not reset, but remains on to indicatea "fault" or "alarm" state.

The hold off circuit 58 generally comprises zener diode 104, transistor106, transistor 108, and resistors 110 and 112. This combinationoperates to disable the reset function if the incoming ac voltage isless than 87 volts across leads 130, 132. The combination of zener diode104 and transistor 106 comprise a voltage regulator which limits thevoltage on the circuit to approximately 12.7 volts. When that voltagelevel is reached, the zener diode 104 will break down and current willflow through it to the base of transistor 106, thus turning ontransistor 106. Resistor 110 is connected to the collector of transistor106 and the base of transistor 108 to bias these transistors. Whentransistor 106 is turned on, transistor 108 switches off. Withtransistor 108 off, the reset timer 64b is enabled and begins tooperate. Accordingly, when there is sufficient voltage across thevoltage regulator, comprised of zener diode 104 and transistor 106,transistor 106 turns on, thereby turning off transistor 108, allowingthe reset timer 64b to operate again. Conversely, the circuit will holdoff or delay the reset operation until there is at least 12.7 volts dcacross resistor 126, which corresponds to 87 volts on the incoming leads130 and 132. Thus, the hold-off circuit prevents the indicator frombeing reset if there is not at least 87 volts available to indicatorassembly 14 from control power transformer 140. The hold-off circuitalso prevents low voltages and stray noise from causing a false reset ofthe indicator.

In the example of the preferred embodiment described above, thefollowing electrical components are suitable for use in the circuitry ofthe sensor assembly 12 and indicator assembly 14:

    ______________________________________                                                 Reference                                                            Component                                                                              Number    Manufacturer Description                                   ______________________________________                                        Bridge   28, 124   Diodes, Inc. 1 amp, 400 v,                                                                 Mgf's Part No.                                                                DB104                                         Varistor 30, 81    Panasonic    ZNR (MOV),                                                                    22 v, Mgf's                                                                   Part No.                                                                      ERZ-CO5DK220                                  Programming                                                                            32        Mepco/Centralab                                                                            1/4 W, 5%                                     Resistor                                                                      Transistor                                                                             34, 72, 74,                                                                             Siliconix    FET "N"                                                90, 94, 96             Channel, Mgf's                                                                Part No. 2N7000                               Capacitor                                                                              36, 68    Mepco/Centralab                                                                            Cermic, 0.1 uF,                                                               50 v, 20%, Mgf's                                                              Part No.                                                                      CZ2OC104M                                     SUS      38        Motorola     SUS Transistor,                                                               Mgf's Part                                                                    No. 2N4989                                    LED      40        Motorola     Fiber Optic                                                                   Emitter, Mgf's                                                                Part No.                                                                      MFOE76                                        Resistor 42        Mepco/Centralab                                                                            33 ohm, 1/4 W, 5%                             Resistor 44, 86    Mepco/Centralab                                                                            220 ohm, 1/4                                                                  W, 5%                                         Reed Switch                                                                            50        Hamlin       Form A,                                                                       Mgf's                                                                         Part No.                                                                      MDSR-4-185                                    Receiver 62        Motorola     Fiber Optic                                                                   Detector, Mgf's                                                               Part No.                                                                      MFOD75                                        Dual CMOS                                                                              64        Texas        Mgf's Part No.                                Timer              Instruments  TLC556IN                                      Resistor 66                     27K, 1/4 W, 5%                                Coil     21        Cramer Coil  7,000 Turns of                                                                #41 gauge                                                                     copper wire                                   Thermistor                                                                             152       Fenwal       1,000 Ω at 25° C.,                                               Electronics, Inc.                                                             resistance                                                                    ratio                                                                         6.35 Mfg.'s                                                                   Part No.                                                                      197-102DAG-A01                                Resistor 154       Mepco/Centralab                                                                            825 Ω, 1/4                                                              W, 1%                                         Capacitors                                                                             162, 164  Mepco/Centralab                                                                            1 μF, 50 v                                 Inductor 166       Nytronics    10 μH                                      Resistor 67, 100,  Mepco/Centralab                                                                            10K, 1/4 W, 5%                                         110, 112                                                             Transistor                                                                             70, 92    Motorola     PNP Transistor,                                                               Mgf's Part No.                                                                2N3906                                        Resistor 76, 98    Mepco/Centralab                                                                            470 ohm, 1/4                                                                  W, 5%                                         Indicator                                                                              80        Ferranti     Mgf's Part No.                                                                54NR202                                       Resistor 84        Mepco/Centralab                                                                            680K, 1/4 W, 5%                               Capacitor                                                                              88        Panasonic    Tantalum, 10 uF,                                                              16 v, Mgf's                                                                   Part No.                                                                      ECS-FICE106K                                  Zener Diode                                                                            104       American Power                                                                             12 v, 500 mW, 5%                                                 Devices      Mgf's Part No.                                                                IN5242B                                       Transistor                                                                             106, 108  Motorola     NPN Transistor,                                                               Mgf's Part No.                                                                2N3904                                        Varistor 114       Panasonic    ZNR (MOV),                                                                    200 v, Mgf's                                                                  Part No.                                                                      ERZ-C05DK201                                  Capacitor                                                                              116, 118  Panasonic    Poly. Film,                                                                   0.47 uF, 250 v,                                                               Mgf's Part No.                                                                ECQ-E2474KF                                   Resistors                                                                              120, 122  Mepco/Centralab                                                                            47K, 1/4 W, 5%                                Resistor 126       Mepco/Centralab                                                                            18K, 1/4 W, 5%                                Capacitor                                                                              128       Panasonic    Electrolytic,                                                                 470 uF, 16 v,                                                                 Mgf's Part No.                                                                ECE-A1CU471                                   ______________________________________                                    

While a preferred embodiment of this invention has been shown anddescribed, modifications thereof can be made by one skilled in the artwithout departing from the spirit or teaching of the invention. Theembodiments described herein are exemplary only and are not limiting.Many variations and modifications of the system and apparatus arepossible and are within the scope of the invention. Accordingly, thescope of protection is not limited by the above description, but is onlylimited by the claims which follow, that scope including all equivalentsof the subject matter of the claims.

What is claimed is:
 1. An apparatus for detecting the presence of faultson an electrical conductor; comprising:mean for sensing an overcurrentcondition in the electrical conductor, said sensing means comprising atemperature compensation circuit for negating the effect of temperaturechange on said sensing means; means remote from said sensing means forindicating the presence of an overcurrent condition in the electricalconductor; means for communicating the presence of an overcurrentcondition sensed by said sensing means to said indicating means; meansfor automatically resetting said indicating means; means for disablingthe automatic operation of said resetting means when a predeterminedminimum voltage is not present on the electrical conductor; and saidtemperature compensation circuit comprising a current transformer havinga wire coil, said wire coil having a positive temperature coefficient; athermistor having a negative temperature coefficient; and a resistorconnected in parallel with said thermistor, said resistor having anegligible resistance change at temperatures within the range ofapproximately -40 degrees C. to +85 degrees C.
 2. The apparatus of claim1 wherein said parallel combination of said resistor and said thermistoris connected electrically in series with said wire coil, and wherein theresistance of the parallel combination decreases in proportion to theincrease in resistance of said wire coil when the temperature of saidwire coil and said thermistor is increased.
 3. Apparatus for detecting afault on a monitored electrical conductor, comprising:means for sensinga fault in the conductor, said sensing means including a temperaturecompensation circuit for negating the effect on said sensing means oftemperature changes within the range of approximately -40 degrees C. to+85 degree C., and including a low pass filter circuit for filtering outundesirable high frequency transients induced in said sensing means;means for remote from said sensing means for indicating the presence ofa fault in the electrical conductor; means for communicating thepresence of a fault sensed by said sensing means to said indicatingmeans; means for automatically resetting said indicating means; andmeans for disabling automatic operation of said resetting means unless apredetermined minimum voltage is present on the conductor; saidcommunication means electrically isolating said sensing means from saidindicating means; said resetting means continually resetting saidindicating means at regular predetermined intervals unless disabled bysaid disabling means; said disabling means comprising a hold-off circuitfor deactivating said resetting means when said predetermined voltage isnot present on the conductor; and said temperature compensation circuitcomprising: a current transformer having a wire coil, said wire coilhaving a positive temperature coefficient; a thermistor having anegative temperature coefficient; and a resistor connected in parallelwith said thermistor, said resistor having a negligible resistancechange at temperatures within the range of approximately -40 degrees C.to +85 degrees C.
 4. Apparatus for indicating that a fault of apredetermined current magnitude has occurred in a monitored electricalconductor on the high voltage side of an electrical distributiontransformer, comprising:a sensor for generating a signal proportional tothe current in the monitored conductor; a low pass filter for filteringhigh frequency transients in the signal generated by said sensor toprevent the transients from causing misoperation of the apparatus; anoptic transmitter for transmitting a light pulse when a fault conditionhas been sensed by said sensor; an optic receiver for receiving saidlight pulse and transforming said pulse into an electrical pulse; anindicator for indicating the presence of a fault condition in themonitored conductor upon receipt of said electrical pulse, saidindicator having first and second indication states for, respectively,displaying indications of "normal" or "fault" conditions; a resetcircuit for resetting said indicator from the second of said indicationstates to the first of said indication states; and a hold-off circuitfor delaying the reset circuit from resetting said indicator unless apredetermined minimum voltage is present on the low voltage side of theelectrical distribution transformer.
 5. The apparatus of claim 4 furthercomprising a temperature compensation circuit for compensating fortemperature-induced variations in the signal that is generated by saidsensor.
 6. The apparatus of claim 5 wherein said optic transmittercomprises:a light emitting diode adapted for transmitting a pulse oflight through a fiber optic cable when current flows through said diode;and a silicon unilateral switch which closes and allows current to flowto said light emitting diode when a predetermined voltage is presentacross said switch.
 7. The apparatus of claim 6 wherein said optictransmitter further comprises a resistor for causing said predeterminedvoltage to appear across said silicon unilateral switch when a fault ofthe predetermined current magnitude has been sensed by said sensor. 8.The apparatus of claim 5 further comprising a test circuit for testingsaid apparatus and causing said optic transmitter to transmit a lightpulse to said receiver to indicate the presence of an overcurrentcondition in the monitored electrical conductor, said test circuitcomprising a manually operable switch in said optic transmitter.
 9. Theapparatus of claim 4 wherein said low pass filter has a roll-off pointwithin the range of approximately 1,000-2,000 hz.
 10. The apparatus ofclaim 5 wherein said low pass filter has a roll-off point within therange of approximately 1,000-2,000 hz.
 11. The apparatus of claim 10wherein said sensor comprises a wire coil having a positive temperaturecoefficient and wherein said temperature compensating circuit comprisesa thermistor having a negative temperature coefficient.
 12. Theapparatus of claim 11 wherein:said temperature compensating circuitfurther comprises a resistor having a negligible resistance change attemperatures within the range of approximately -40 degrees C. to +85degrees C. connected in parallel with said thermistor; wherein saidparallel combination of said resistor and said thermistor is connectedelectrically in series with said wire coil; and wherein the resistanceof the parallel combination decreases in proportion to the increase inresistance of said wire coil when the temperature of said wire coil andsaid thermistor is increased.
 13. The apparatus of claim 5 furthercomprising a timer adapted for receiving the electrical pulse from saidoptic receiver wherein said timer, upon receipt of the electrical pulsefrom said optic receiver, transmits an output pulse causing saidindicator to change from the first of said indication states to thesecond of said indication states and causing said reset circuit to delaya predetermined period of time before resetting said indicator.
 14. Anapparatus for detecting the presence of faults on an electricalconductor, comprising:a sensor for sensing an overcurrent condition inthe electrical conductor, said sensor comprising a temperaturecompensation circuit for negating the effect of temperature change onsaid sensing means, said temperature compensation circuit comprising: acurrent transformer having a wire coil, said wire coil having a positivetemperature coefficient; a thermistor having a negative temperaturecoefficient; and a resistor connected in parallel with said thermistor,said resistor having a negligible resistance change at temperatureswithin the range of approximately -40 degrees C. to +85 degrees C.; anindicator for indicating the presence of an overcurrent condition in theelectrical conductor; and a signal transmitter for communicating thepresence of an overcurrent condition sensed by said sensor to saidindicator.